1. Field of the Invention
The invention relates to geophysics, namely, to methods of locating useful minerals by seismic prospecting and prospecting by electromagnetic waves, and more particularly it relates to geophysical prospecting of polymineral ore bodies.
2. Description of the Prior Art
Ore bodies in different deposits, and even within a single deposit, may differ broadly in their mineral composition. In any deposit, only those ore bodies which contain a substantial percentage of the useful mineral or a combination of several useful or valuable minerals are of commercial attraction. Hence, the determination of the mineral composition of ore bodies represents an essential aspect of geophysical prospecting.
Conventionally, the determination of the mineral composition of ore bodies has been conducted by a geological technique including drilling a network of holes through a deposit and assessing the contents of the ore bodies traversed by the holes by analysis of core samples taken therefrom. However, as the mineral composition of ore bodies tends to vary even within one and the same ore body, it becomes necessary to drill multiple holes to adequately evaluate the feasibility of opening up this or that ore body by mine workings and its full-scale mining. This geological approach is relatively time- and labor-consuming and costly, and even then it is not sufficiently effective, for each single hole supplies data on the composition of the ore body within a very limited area, more often than not only a few centimeters in diameter, which does not permit assessment with appropriate accuracy of the average composition of the ore body and the total amount of a valuable mineral contained therein.
There are also known several geophysical methods of evaluating the mineral composition of polymineral ore bodies. There is known a contact method of polarization curves (cf., Y.S. Ryss, Prospecting of Ore Bodies by Contact Method of Polarization Curves, in Russian, published in 1973 by NEDRA Publ., Leningrad, p. 167) based on connecting one pole of an electric current source to a point of the ore body in a hole or in a mine working, and connecting the other pole of the same source to an electrode embedded in the parent rock, whereafter the electric current is made to flow through the circuit thus formed, the value of this current being gradually raised. Various minerals contained in the ore body under investigation enter electrochemical reactions at different values of the flowing current, so that the value of the registered potential varies in steps. The values of the potentials of the electrochemical reactions are compared with reference values obtained in advance in a laboratory environment and represent the presence in the ore body of a known mineral. Thus, when the registered potential values coincide with the reference ones, a conclusion can be made on the actual presence of the corresponding mineral in the ore body. In this way the mineral composition of the ore body under investigation is analyzed; whereas the threshold values of the electric current are used for assessing the quantitative composition of the minerals and the approximate scale of the ore deposit, as part of the phase of evaluating detected ore manifestations. However, this method involves numerous technical and practical difficulties arising from the complexity of selecting a reliable spot of contact with the ore body, from large values of the electric current required (as big as several dozen to several hundred Amperes). Moreover, the time taken to obtain a single polarization curve is from several dozen minutes to several days, so that one contact session with the ore body usually consumes a number of working days. It can be seen from the above that the employment of this method in an underground mine environment is severely hindered.
The abovedescribed method does not allow prospecting for an ore body unless the latter is pierced by holes or mine workings requiring as it does a direct contact with the ore. Neither is it operable for simultaneously studying several ore bodies, particularly those adjoining one another.
There further exist geophysical methods of propsecting for polymineral ore bodies without traversing them by holes or exploration workings, providing for substantial economy of both time and costs in exploring ore bodies for assessing the feasibility of their full-scale mining.
There is known a method of geophysical prospecting for polymineral ore bodies (of. the USSR Inventor's Certificate No. 330,411; Int.Cl. G 01 V 11/00, published in "Discoveris, Inventions, Industrial Designs, Trademarks" Gazette on Feb. 24, 1972) employed for detecting pockets of piezo-optical quarts. The method includes exciting elastic waves in the rock block housing the ore bodies, registering the electromagnetic pulsed radiation generated by the ore bodies under the action of these elastic waves in the form of pulse bursts, each burst corresponding to one ore body, subjecting to Fourier transform a pulse from the pulse burst corresponding to the ore body under investigation to obtain the pulse height-frequency spectrum of the pulse of the electromagnetic radiation, and carrying information on the mineral composition of the investigated ore body.
There is also produced the amplitude-frequency spectrum of the elastic wave in the range to 30 kHz, whereafter the two spectra are transformed to the same maximum amplitude, the spectrum of the elastic wave is subtracted from the spectrum of the electromagnetic radiation pulse, and the absence or presence in this differential spectrum of the high-frequency portion (to 30 kHz) is considered as an evidence, respectively, of the absence or presence of a piezo-optical quartz pocket in the ore body.
Quartz crystals generate electromagnetic signals only when the amplitude-frequency spectrum of the initiating elastic wave includes frequencies corresponding to the resonance frequencies of quartz crystals. However, due to absorption of the high-frequency part of the elastic wave spectrum, signals in the vicinity and above 30 kHz would not be registered at distances in excess of 20-30 m from the wave source, whereby this known method is impractical for employment for determination of the mineral composition of polymineral ore bodies at distances in excess of 30 meters.
Moreover, the described method of geophysical prospecting for polymineral ore bodies can be used exclusively for detecting quartz crystals and is not suited for determining the mineral composition of complex ore bodies incapable of the piezoelectric effect.